1. Field of the Invention
The present invention relates to an arc tube for a discharge lamp unit, and in particular, to a mercury-free arc tube for a discharge lamp unit that uses a buffer metal halide as a buffer substance instead of mercury, providing an arc tube that is not as harmful to the environment as the related art mercury-based arc tube.
2. Description of the Related Art
FIG. 3 shows a discharge bulb which is a related art discharge lamp unit used as a light source of an automotive lamp. The discharge bulb has a structure in which an arc tube 2 having a closed glass bulb 2a as a light emitting portion is unified with an insulating plug body 1 made of synthetic resin. The arc tube 2 is gripped at its rear end portion by a metal support member 8 fixed to the insulating plug body 1, and the arc tube 2 is supported at its front end portion by a metal lead support 9 which also serves as a current conduction path extended out from the insulating plug body 1.
The arc tube 2 has a structure in which the main light emitting metal halide, the buffer mercury, and the starting rare gas are enclosed in the closed glass bulb 2a which is held between pinch seal portions 2b and 2b located at the opposite ends of the closed glass bulb 2a. Light is emitted by an arc generated through electric discharge between the pair of electrodes 3 and 3. The arc bulb is superior in a light-emitting amount to an incandescent bulb, and has a long lifetime. For this reason, nowadays there is a tendency for this type of discharge bulb to be used as a light source for a head lamp or a fog lamp of an automobile.
Reference numeral 4 designates a lead wire led out from the pinch seal portion 2b. Reference numeral 5 designates a molybdenum foil for connecting a tungsten-made electrode 3 to the lead wire 4. Further, the arc tube 2 is integrally welded to an ultraviolet-shielding shroud glass 6 to thereby form a structure in which the closed glass bulb 2a is surrounded by a closed space sectioned by the shroud glass 6. This configuration cuts-off ultraviolet rays in a wavelength range harmful to the human body from a light emitted from the arc tube 2, and concurrently keeps the closed glass bulb 2a at a high temperature.
In a related art closed glass bulb 2a, mercury is enclosed, but it is known that mercury is toxic to the environment. In response to the social needs of reducing causes of global environmental pollution as much as possible, it is desirable to develop a mercury-free arc tube, i.e., an arc tube not containing the mercury that is toxic to the environment.
The following findings have been obtained in the process of research and development on a mercury-free arc tube.
In the discharge lamp unit (the discharge bulb), current applied to the arc tube is adjusted by a ballast such that the arc tube is lit at rated power (for example, 35 W). The mercury enclosed in the closed glass bulb acts mainly as a buffer substance which helps the arc tube maintain a predetermined tube voltage (for example, 85 V) for rated power (for example, 35 W) to reduce damage to the electrodes due to increase of the tube voltage (increase of electron number colliding with the electrodes). The mercury acts also as a light emitting substance for emitting a predetermined emitting light (white light) in cooperation with the main light emitting metal halide. For this reason, if the mercury is removed from the closed glass bulb, the following changes (problems) occur in the characteristics as the arc tube.
First, the tube voltage goes down. Accordingly, the current applied to the discharge lamp unit (the discharge bulb) therefore increases, resulting in reducing damage to the electrode or a light emitting efficiency (the rate of the light emitting amount to the applied current).
Second, a light flux goes down by an amount that light emitted by the mercury is absent in a visible range.
Third, the color of emitted light is different (reddish) from the color of light emitted from the conventional mercury-containing arc tube.
The inventor selected a metal halide as a substitute for the mercury (and a non-toxic substance to the environment), sealed the metal halide in the closed glass bulb, and performed studies as to whether characteristics near to those of the related art mercury-containing arc tube could be obtained or not. As a result, the inventor discovered a mercury-free arc tube having similar lighting properties as that of the related art mercury arc-tube.
The invention is based on the problems in the prior art and the inventor""s knowledge, and it is an object of the invention to provide a mercury-free arc tube for a discharge lamp unit in which characteristics near to or better than those of the related art arc tube can be obtained.
For accomplishing this object, according to a first aspect of the invention, a mercury-free arc tube for a discharge lamp unit comprises: a closed glass bulb held between pinch seal portions located at opposite ends of the closed glass bulb; and a pair of electrodes provided in the closed glass bulb so as to be opposite to each other, the closed glass bulb including a main light emitting metal halide, a buffer metal halide, and a starting rare gas enclosed in the closed glass bulb; wherein the buffer metal halide has ionization potential of about 5 to 8.5 eV and excitation potential of about 4.5 eV or less having emission spectrum in a visible range.
(Working)
In substitution for the related art mercury, the buffer metal halide enclosed in the closed glass bulb prevents a large reduction of the tube voltage and light flux owing to the bulb not containing any mercury, and contributes to dissolution of the first and second problems discussed above.
In addition, if at least one kind of the metal halides emitting light near to the light emissions of the mercury is selected to be enclosed in the closed glass bulb, reduction of the amount of emitting light or of the light flux is complemented in the visible range (the white light), which contributes to dissolution of the third problem discussed above.
However, if ionization potential of the buffer metal halide is too low (less than 5 eV), since electrons are emitted at low energy and the tube voltage does not rise sufficiently, and reversely, if ionization potential is too high (more than 8.5 eV), discharge is difficult to be stable by the amount that ionization is difficult, and the light emissions are less and the light flux does not rise sufficiently. Therefore, it is desirable that ionization potential of the buffer metal halide is within a range between about 5 and 8.5 eV where the tube voltage and the light flux are sufficient for lighting purposes.
Although ionization potential of the buffer metal halide ranges from about 5 to 8.5 eV, if excitation potential of this substance is too high (more than 4.5 eV), it is not ready for coming to emission, and therefore it is desirable that excitation potential of the buffer metal halide has an emitting spectrum being not so high but below 4.5 eV.
In a second aspect of the invention, in the mercury-free arc tube for a discharge lamp unit as set forth in the first aspect, the buffer metal halide may comprise one or more halides including Ta, Th, Cu, Rb, Nb and Pd. In a preferred embodiment, at least one of the one or more halides is Ta, Cu, Rb, or Nb.
(Working)
Substances used in the related art case may similarly be used as the main light emitting metal halide and the starting rare gas in the present invention. That is, the main light emitting metal halide may be a sodium-scandium based halide such as NaI and ScI3, and the starting rare gas may be Xe.
The buffer metal halide used in place of the mercury is at least one kind selected from the group of Ta, Th, Cu, Rb, Nb and Pd. When the buffer metal halide is enclosed in the closed glass bulb, the great reduction in the tube voltage and the light flux caused by not using mercury can be suppressed, contributing to dissolution of the first and second problems.
In the preferred embodiment, at least one of the halides may be selected from halides of Ta, Cu, Rb, and Nb emitting light near to that of mercury and enclosed in the closed glass bulb. Therefore, the reduction of the amount of emitting light or of the light flux is complemented in the visible range (the white light), contributing to dissolution of the third problem.
In a third aspect of the invention, the mercury-free arc tube for a discharge lamp unit as set forth in the first and second aspects is structured such that a weight ratio of the buffer metal halide to the main light emitting metal halide is about 1 to 500%.
(Working)
If the weight ratio of the buffer metal halide to the main light emitting metal halide is less than about 1%, the light emission by the buffer metal halide is not sufficient and the light flux rises insufficiently. Reversely, being more than about 500%, the buffer metal halide is too much and impedes the light emission of the main light emitting metal halide, and so the light flux rises insufficiently. Therefore, desirably the weight ratio of the buffer metal halide to the main light emitting metal halide is about 1 to 500%.